The invention relates to electronic audio signal systems and devices. The invention also relates to digital communications.
Data compression is extremely important to the music industry. In digital audio signal systems, digital samples of sound are stored on a Compact Disk Read Only Memory (CD ROM). Fidelity of the sound is proportional to the rate at which the sounds are sampled (the sampling rate) and the number of bits comprising each sample. An audio signal sampled 22,000 times per second (22 kHz) by a 16-bit analog-to-digital converter (ADC) is of far higher fidelity than an audio signal sampled at 11 kHz by an 8-bit ADC. An audio signal sampled at 44 kHz by a 24-bit ADC is of even higher fidelity. However, the 44 kHz, 24-bit sampling produces three times as much data as the 22 kHz, 16-bit sampling and twelve times as much data as the 11 kHz, 8-bit sampling. This is where data compression is so important. The data compression reduces the amount of data stored on the CD ROM, but maintains the fidelity of the sound. Data compression allows an audio signal sampled at 44 kHz by a 24-bit ADC to be stored economically on a CD ROM.
Data compression is also important to the television industry, especially with the emergence of direct broadcast television. In a direct broadcast system, digital signals of near-perfect video images and audio waveforms are encoded according to a known standard, transmitted to a satellite orbiting the earth, and relayed by the satellite on the Ku band to any home equipped with a small dish antenna and a receiver unit. Data compression reduces the amount of video and audio data that must be transmitted.
One compression standard becoming widely used is the MPEG standard. MPEG was established by the Moving Pictures Experts Group of the International Standardization Organization to specify a format for the encoding of compressed full-motion video and audio. MPEG audio compression produces CD quality audio at very high compression rates.
On occasion, errors occur during data transmission or retrieval, so that the audio cannot be properly restored. The errors can affect an entire audio frame, or only portions of a frame. The errors include decode errors (e.g., illegal bit combinations), transmission errors (failed CRC checks on sensitive portions of a frame) and reconstruction errors (a frame cannot be reconstructed by the required time because a buffer runs out of data). These errors can distort the sound over the speakers.
The errors can be concealed by most audio decoders. The most common method of error concealment among MPEG Audio Decoders is simply to throw out the audio frame with the error, and jump ahead to the next frame. The decoder's output in response to a Delete.sub.-- Frame signal is shown in FIG. 1a. One problem with this method is that a discontinuity is introduced where a bad frame is removed. The discontinuity is almost always audible. A second problem is that the audio decoder might not be able to find another good frame with which to re-establish synchronization in the required time. The second problem is more likely to occur when the audio decoder has no control over the incoming data rate, as in cable and satellite feeds. It can be an even bigger problem in combined audio/video systems since so little buffer space is reserved for the audio data. Yet a third problem, which also arises in combined audio/video system, is synchronization of the audio and video signals. Skipping an audio frame destroys synchronization with the video presentation. Restoring proper synchronization introduces additional discontinuities.
Another method of concealing audio errors is replacing a bad audio frame with a previous good frame. The decoder's output in response to a Bad Frame(s) signal is shown in the FIG. 1b. The advantage here is that synchronization with the video presentation is maintained. However, two problems arise. First, extra hardware (about 11.7k bits of memory) is required to store the data necessary to replay the previous audio frame, and this means added cost. Second, repeating the last frame might sound quite objectionable, especially if it needs to be repeated many times.
A third method of concealing audio errors is freezing the audio data until good audio data can be decoded. The decoder's output in response to a Freeze.sub.-- on.sub.-- Error signal is shown in the FIG. 1c. This method also allows synchronization with the video presentation to be maintained. It also avoids the insertion of bogus data to replace bad frames. However, the error concealment is quite noticeably audible (as an abrupt mute), especially when the freeze lasts at least one frame or more.